Ionization fire alarm system



Nov. 14, 1967 E. MElLI ETAL IONIZATION FIRE ALARM SYSTEM Filed NOV. 12. 1963 INVENTORS United States Patent 3,353,170 IQNIZATION FIRE ALARM SYSTEM Ernst Meiii, Kusnacht, and Thomas Lampart, Mannedorf, Switzerland, assignors to Cerberus AG, Mannedorf, Switzerland, a corporation of Switzerland Filed Nov. 12, 1963, Ser. No. 322,775 Claims priority, application Switzerland, Nov. 19, 1962, 13,526/ 62 5 Claims. (Cl. 340-237) This invention relates to an improved method and apparatus responsive to changes in the composition of a gas, and more particularly to an ionization type fire alarm for testing air for the presence of smoke and other combustion products.

Ionization type detectors or fire alarms for detecting the presence of smoke and combustion products have the advantage that they detect the presence of combustion during its earliest stages or during what is known as the incipient period when combustion gases alone are being produced by the combustion. On the other hand, fire detection devices of the thermostatic or photoelectric type only respond to the later stages of the combustion when the fire has become well established. It is obvious, therefore, that a fire or combustion detection device which operates upon the ionization principle is much more effective in providing an early warning of the presence of combustion than detection devices of other types.

Ionization type combustion detection devices are known per se and are shown, for example, by United States Patent No. 2,465,377, issued to Walter C. Jaeger on Mar. 29, 1949, and United States Patent No. 2,702,898, issued to Ernst Meili, co-inventor herein, on Feb. 22, 1955. Generally, these devices comprise at least one ionization chamber connected to a current intensification element such as a cold cathode tube or transistorized amplifier. Where two ionization chambers are employed one usually is a test ionization chamber which is open to the atmosphere in which the combustion gases are detected and in which there is a small quantity of radium which emits alpha particles to render the air in the ionization chamber conductive by ionizing air molecules in the chamher. The test ionization chamber is connected electrically in series with a reference or comparison chamber which also contains radium. The reference chamber generally is constructed in such a manner as to prevent the entrance therein of the combustion products or smoke particles. The reference chamber is in effect a resistance element electrically connected in series with the test chamber.

The indication of the ionization detector depends upon a decrease in the ionization current. As this current is very small, extremely sensitive means are required for its indication. As stated above, a cold cathode tube is preferred for this purpose as it may be maintained in a state of readiness for operation without a heating current, although other means, such as bridge circuit connected to two chambers, may also be employed. Generally, in this situation the pair of adjacent chambers are equally accessible to products of combustion and are so related that a change in the conductivity of the atmosphere in one chamber relative to the other causes an imbalance in the bridge which, in turn, actuates a suitable alarm or signal ling device. In order to trigger the cold cathode tube the change in the ionization current is converted into a change in voltage by connecting the second ionization chamber or resistance equivalent in series with the test chamber. A potential is applied to the series arrangement in a parallel connection with the cold cathode tube and the potential across the test chamber is used for controlling the tube.

In the presence of visible or invisible smoke particles in the atmosphere being monitored, the smoke particles entering the open test ionization chamber adhere to the 3,353,170 Patented Nov. 14, 1967 air molecules. This results in a decrease of ionization current in the presence of combustion gases caused by the incipient fire. Since the ionization chamber is connected electrically in series with the reference chamber across a direct current volage source, a change in ionization current in the test chamber causes a redistribution of the voltage across the reference chamber and the ionization chamber, which always must equal the total applied voltage. Connecting the control electrode of a gaseous discharge tube, a cold cathode tube, at the electrical junction between the test chamber and the reference chamber, permits the change in potential at the point of connection due to the redistribution of voltages to trigger the firing of the cold cathode tube which, in turn, is used to activate a suitable signalling device which indicates the presence of the combustion gases and the incipient fire.

, Regardless of the particuular type of ionization device, a particular disadvantage associated with such equipment is the fact that an open ionization chamber, serving as a measurement or test chamber, is too easily exposed to impurities contained in the surrounding air. The dust layer which form on the apparatus gradually reduces the ionizing radiation. This results in the production of fewer ions and electrons which, in turn, causes reduction in the ionization current and premature actuation of the signalling device. Further, the dust layer, with a greater or smaller insulation effect, when situated on the electrodes of a chamber, decreases the intensity of the electric field, with the disadvantageous result that the ionic current is also reduced and again a premature actuation of the signalling means occurs. In any case, the settlement of dust or dirt on the ionization chamber produce an unfavorable effect on the stability of the ionic current and thereby on the alarm system as a whole.

It has also been found that dirt or dust settlement on the protective screen of a chamber also adversely affects the functional performance of the ionization device since the screen openings may be partly or fully clogged, thus preventing the atmosphere to be measured from freely entering the chamber. This situation also substantially reduces the effectiveness of the device.

An obvious solution to the problems set forth would be a periodic cleaning of the apparatus. Such a solution has frequently been implemented, but it has been found to be an impractical one. It has been found that this procedure is time consuming and costly since the ionization type alarm device often is located in diflicultly accessible areas. Efforts, heretofore, to find a means to substantially reduce or completely eliminate the occurrence of dust settlement have resulted in the provision of a fine filter through which the atmosphere enters the device, as set forth for instance in our copending United States application, Serial No. 90,989, filed Feb. 23, 1961, now US. Patent 3,154,773, granted Oct. 27, 1964. The fine filter permits entry of combustion gases while blocking the substantially larger dust particles. However, this method is then generally only employed in actual practice with very dusty spaces, for general application it has been found to be too uneconomical because a great deal of accessory equipment is necessary.

While it will be recognized that a certain amount of dust is present in most atmospheres that need be monitored, a particular disadvantage is associated with the use of present ionization type devices. This disadvantage is occasioned from the utilization in known devices of radioactive substances which have a large sphere or radius of influence. The drawback lies in the fact that the beta and gamma rays emitted in the process ionize a large area outside the ionization chambers, whereas on the other hand the ionization sphere of the alpha rays is potentially limited to the chambers. The beta and gamma rays, however, because of their longer sphere of influence and their stronger penetration characteristics, generally possess an ionization effect in the air of more than two meters. Because of this ionization, dust particles suspended in the air are charged with electricity which is partly transferred to the alarm system via the existing electric field, i.e., the dust particles settle down on the screen and the electrodes as well as the radio-active apparatus of the chamber, thus producing the disadvantages noted above.

It is, therefore, a principal object of the present invention to provide a combustion gas detection device of the ionization type which is free from the disadvantages described above.

It is another object of the invention to provide a combustion gas detection device of the ionization type which substantially eliminates ionization outside the ionization chamber.

Yet another object of the invention is to provide an improved combustion gas detection device for fire alarm of the ionization type which is highly reliable in operation.

A further, object of the invention is to provide in combination with an ionization type combustion chamber a convenient means for compensating for the normally disadvantageous effects of atmospheric dust present in a monitored space.

Other features, objects and advantages of the invention will become apparent by reference to the following detained description and drawings in which:

FIGURE 1 shows schematically a circuit diagram of a detecting apparatus or fire alarm of the ionization type which conveniently can be used by way of example in association with the instant invention; and

FIGURE 2 schematically illustrates a modified form of shielding or dome for the ionization chamber of the apparatus of FIGURE 1.

In the achievement of the aforementioned objects there is provided, in accordance with the instant invention, an improved fire alarm incorporating such a source of ionization and which cooperates with the ionization chamber such that, it does not produce, as generally considered with respect to normal atmospheric conditions i.e., 760 millimeters mercury pressure and room temperature, more than one ion pair/ cubic millimeter per second at a disstance of centimeters from the outer limits of the ionization chamber. Such cooperative relation between the ionization source and the ionization chamber can be achieved, according to the invention, by selecting an ionization source which due to its characteristics, as to be more fully explained shortly, of itself fulfills the desired ionization requirement, or, if such is not completely the case,,then suitable and very simple expedients, such as relatively uncomplicated constructed screen means or the like can be provided to ensure that the desired ionization effect is attained, and without impairing the operational reliability and integrity of the fire alarm.

As background material it should be appreciated that a reduction of the external ionization to the aforementioned value with previously employed conventional radioactive materials, in particular radium, would only be possible in the presence of considerable disadvantages for example by providing very heavy and costly screening or by reducing the radiation source to a value resulting in unreliable operation of the fire alarrn-because the external ionization due to beta and gamma radiation is quite pronounced in comparison with the short alpha radiation useful for the ionization chamber. Due to this fact it is readily understandable that the heretofore known fire alarms in their practicalconstructions, produced an ionization effect greater than mentioned above, in fact considerably larger than one ion pair/cubic millimeter per second at a distance of 10 centimeters from the outer limits of the ionization chamber. Furthermore, it is known that radium heretofore employed as the radioactive material in known fire alarms, at a distance of 0.5 centimeter therefrom produces about 10' times more 4 ion pairs/cubic millimeter than it does at a distance of 10 centimeters.

On the other hand, the ionization sources according to the instant invention, however, and which permit obtaining the herein-mentioned desired ionization effect in relatively simple manner, produce at least 10 times more ion pairs/cubic millimeter at a distance of 0.5 cm. from the,

source than they do at a distance of 10 cm. The larger the ratio of ionization between short and long radiation,

the greater the ionicicurrent can be without exceeding the nium, namely P0 208 and P0 209 can be used for example, or, in fact, any radioactive substance fulfilling the requirements noted heretofore. These radio-active isotopes are characterized by emitting either no beta or gamma radiation or, if they do, such beta or gamma radiation is small in comparison with alpha radiation and is substantially ineffective in ionizing the air molecules and, thus in producing a disadvantageous settlement of ionic-currentinterfering dust.

According to another embodiment of the invention a source of ionization which emits beta rays characterized by a penetration length confined to the limits set forth above may also be used. Representative sources of such a material are tritium and carbon-l4.

The use of either an alpha emitter with weak beta or gamma rays or a pure beta emitter whose rays are of short influence gives totally unexpected favorable results over existing procedures and devices which employ radium, especially when an alarm system in which they are used is equipped with a cold cathode tube. For instance the ionization within the tube is lower than when radium is used and while the beta and gamma rays emitted by radium tend to penetrate the glass of the tube, those emitted by the ionization sources in accordance with this invention are substantially insulated. Thus ionization of the gas space of the tube is substantially avoided thereby inhibiting a current between the tube electrodes which maintains the orientation sensitivity of the tube-a phenomenon not achieved when radium is used.

If there appears in the envelope or capsule for the radioactive material a bremsstrahlung (electromagnetic radiation) which is produced by the primary nuclear emission, or, if with the mentioned isotopes there appears beta and gamma rays poor in energy then their effect canbe further diminished through the use of suitable screen means. The screening of such beta rays as well as electromagnetic radiation is relatively easy because in comparison with the beta and gamma rays of radium-as mentioned-one is concerned with rays which are weak in energy.

Referring now to FIGURE 1 a combustion gas detection device or fire alarm of the ionization type suitable for use according to the instant invention is shown. The ionization or testing chamber 1 includes a generally circular grid-like screen or dome 2 acting as one of its electrodes. It is here to be mentioned that in the event beta and electromagnetic radiation are to be screened, for reasons previously mentioned herein, then in accordance with the invention and as shown in FIGURE 2 a modified form of screen or dome 2 can be used. By way of example, such dome 2' is shown to embody a pair of superimposed and spaced grid-like screens 20 and 21 which are offset with respect to one another to provide a labyrinth passage arrangement for enabling the combustion products or smoke particles to enter the ionization chamber 1 while screening the beta and electromagnetic radiation. The plate 3 provides the other. electrode for the ionization chamber 1. On the face of this electrode plate 3 confronting the grid-like dome 2 there is fixed the radioactive source 4, of any type capable of meeting the requirements noted herein,

and which is advantageously enclosed in known manner in a suitable casing or encapsuling means 18.

Further, a resistor 5 is connected in series with the ionization chamber 1 and, in this illustrative embodiment, a cold cathode tube 6 incorporating an anode 7, cathode 8 and control electrode or grid 9 is connected via the latter to the junction point 10. A suitable direct current supply is applied across the cold cathode tube 6. Moreover, a relay 11 for actuating a suitable alarm or signaling device 12 is electrically coupled with the cold cathode tube 6.

The aforedescribed apparatus generally functions in the following manner: Under normal circumstances, namely in the absence of combustion gases or smoke particles the ionization current flowing through the resistor 5 is sufiicient to prevent ignition of the cold cathode tube 6. Now, if however, combustion gases enter the ionization chamber 1 the ionization current is reduced and the voltage drop across the resistor 5 becomes smaller, to an extent suflicient to permit ignition of the cold cathode tube 6, in known manner. As a result, the relay 11 is actuated and, in turn, the signaling device 12, in order to appropriately indicate the presence of the detected products of combustion in the thus monitored atmosphere.

The foregoing disclosure and drawing are merely illustrative of the principles of the invention and are not to be interpreted in a limiting sense, For instance, particular physical structure of the apparatus chosen to illustrate the invention can be replaced with any of the well known ionization type detectors. The only limitations are to be determined from the scope of the appended claims.

We claim:

1. In a fire alarm of the ionization type responsive to changes in the composition of a gas comprising at least one ionization chamber open to the gas to be tested, means for ionizing the gases in said ionization chamber, the improvement which comprises said ionizing means incorporating a radiation source which cooperates with said ionization chamber for inducing under normal atmospheric conditions a gas ionization rate of not more than one ion pair per cubic millimeter per second at a distance of 10 centimeters from the external limits of said ionization chamber, said radiation source being a substance Which is poor in gamma radiation and producing at least 10 times more ion pairs per cubic millimeter at a distance of 0.5 centimeter from said radiation source than at a distance of 10 centimeters, to thereby decrease the effects of dust upon the fire alarm.

2. In a fire alarm according to claim 1 wherein said radiation source is a radio-active material selected from the group comprising Pu 238, Pu 239, Pu 240, Pu 242 and Am 241.

3. In a fire alarm according to claim 1 wherein said radiation source is selected from the group comprising P0 208 and P0 209.

4. Ina fire alarm according to claim 1 wherein said radiation source comprises a short range beta emitter.

5. In a fire alarm according to claim 4 wherein said short range beta emitter is selected from the group comprising tritium and carbon-14.

References Cited UNITED STATES PATENTS 2,408,051 9/1946 Donelian 340-237 2,809,317 10/1957 Meili 340-237 X 3,233,100 2/1966 Lampa-rt 340237 X OTHER REFERENCES Gaynor, F.: Pocket Encyclopedia of Atomic Energy, published 1950, Philosophical Library Inc., N.Y., pp. 17, 18, 82, 100, 102. Copy in Group 220.

Price et al.: Radiation Shielding, published 1957, Pergaman Press, New York, N.Y., p. 11. Copy in Scientific Library.

NEIL C. READ, Primary Examiner.

D. MYER, Assistant Examiner. 

1. IN A FIRE ALARM OF THE IONIZATION TYPE RESPONSIVE TO CHANGES IN THE COMPOSITION OF A GAS COMPRISING AT LEAST ONE IONIZATION CHAMBER OPEN TO THE GAS TO BE TESTED, MEANS FOR IONIZING THE GASES IN SAID IONIZATION CHAMBER, THE IMPROVEMENT WHICH COMPRISES SAID IONIZING MEANS INCORPORATING A RADIATION SOURCE WHICH COOPERATES WITH SAID IONIZATION CHAMBER FOR INDUCING UNDER NORMAL ATMOSPHERIC CONDITIONS A GAS IONIZATION RATE OF NOT MORE THAN ONE ION PAIR PER CUBIC MILLIMETER PER SECOND AT A DISTANCE OF 10 CENTIMETERS FROM THE EXTERNAL LIMITS OF SAID IONIZATION CHAMBER, SAID RADIATION SOURCE BEING A SUBSTANCE WHICH IS POOR IN GAMMA RADIATION AND PRODUCING AT LEAST 108 TIMES MORE ION PAIRS PER CUBIC MILLIMETER AT A DISTANCE OF 0.5 CENTIMETER FROM SAID RADIATION SOURCE THAN AT A DISTANCE OF 10 CENTIMETERS, TO THEREBY DECREASE THE EFFECTS OF DUST UPON THE FIRE ALARM. 